The present invention relates to controls that operate transducers. In particular to controls that operate transducers that if erroneously active may result in hazardous operation but are always safe when inactive. In this application, a transducer is a device whose function is to turn the electrical energy into some other energy form. As the operation of the transducers may also be suspended by external switches or circuits the present invention is related to devices that include override(s).
Controls operate transducers using switching means hereinafter referred to as a switch, that may be a switch, relay, transistor, other solid state switch, or the like. Controls use either open or closed loop systems. In an open loop system any failure of switch goes undetected. True closed loop systems monitor the output of transducers, indirectly this may react to a switch in an erroneous state after the fault affects output but it is an expensive and less reliable means to detect switch failures. Further unless the feedback results in opening every switch in a transducer circuit, a switching failure(s) can still produce erroneous operation while the circuit still has a functional switch since the faulty switch(s) could be the one the feedback uses to alter operation.
U.S. Pat. No. 5,760,493 addresses the problem of improper transducer operation in appliances. The method does not detect faults. The intention of U.S. Pat. No. 5,760,493 is to limit improper transducer operation should one of its switches fail. In this approach two switches must be closed to permit transducer operation. If one switch shorts the transducer is erroneously active when the functional switch is closed to operate another transducer. This method only prevents improper operation during part of a cycle. Since faults are not detected, operation continues allowing the possibility of additional faults developing, further compromising safe operation.
U.S. Pat. No. 5,760,493 points out that switching faults are not uncommon when driving large inductive loads. The appliance industry has relied on a plurality of switches to ensure that operation of transducers is stopped at least at the end of a cycle. This plurality of switches includes overrides which directly switch the transducers. However the plurality of switches adds to the cost and reduces overall system reliability.
U.S. Pat. No. 4,866,955 uses switching means to prevent an appliance from operating if its door lid switch has not been opened since the end of the last cycle. The intended state of the switch is not known, it is assumed that the switch must open between cycles. The control is therefore incapable of stopping the appliance should the switch fail to open as intended during a cycle. The prior art shown in U.S. Pat. Nos. 3,367,089, 4,307,392, 4,951,037, shows methods of detecting switching faults that affect transducers. The transducers are display elements. U.S. Pat. Nos. 4,307,392 and 4,951,037 determine the functionality of the transducer and any fault causing any switch to be in an erroneous state. U.S. Pat. No. 3,367,089 only detects faults affecting the switches. These approaches apply only to switching means operated solely by the control.
The application of transducers in these patents can produce unsafe conditions when erroneously unenergized. For this reason the methods in these patents seek to detect erroneously unenergized transducers in addition to detecting erroneously energized transducers. Additionally U.S. Pat. Nos. 4,307,392 and 4,951,037 seek to verify the functionality of the transducer. These are burdens the present invention does not share. In the applications of the present invention a fault in the switching means or its transducer causing it to be inactive does not pose a safety hazard.
In the display verification patents no method is presented to automatically stop hazardous transducer operation. The approach is to signal the operator that a fault has been found to prevent a fault from causing a hazard. If the operator does not receive the fault indicating signal, operation remains potentially unsafe. While the control of the present invention may signal the operator that a fault has been detected, if the operator fails to observe this signal operation is still safe because the control halts transducer operation.